1. Field of the Invention
This invention relates to an emulsion adhesive composition for use in bonding wood and other substrates.
2. Description of the Prior Art
In the bonding of wood and other porous substrates, an emulsion adhesive composition based on a polyvinyl acetate emulsion has been commonly employed. Bonding by means of such an adhesive is effected by applying the adhesive to a plurality of members of wood or the like and pressing them together between a pair of metal platens. In this process, the emulsion adhesive is squeezed out from the glue line and cures between the substrate and the metal platens so that withdrawal of the product from the platens at completion of bonding is often difficult. Moreover, the adhesive squeezed out from the glue line at pressing sticks to the platens and cures in situ so that it takes much time and labor to remove the cured adhesive and clean the surfaces of the platens.
These problems are particularly onerous in finger joint work in which the glue line comes in direct contact with the platens, edge gluing in the manufacture of laminated wood, and veneer lamination and other works where the adhesive may strike through the thin members. Moreover, the problem of sticking and curing of the adhesive occurs not only with the platens mentioned above but also with other spreaders or coating machines used for the application of such adhesives, and a great deal of effort is required in removing the cured masses of splashed or spilt adhesive.
To overcome these problems, it has been proposed to coat the working surface of the press or coater directly with a surface lubricant such as silicone, wax, lecithin or the like. When such a provision is made, removal of the cured adhesive is facilitated by the action of the lubricant but the application of a surface lubricant means the addition of one new process step to the production line and, hence, an increase in production time as well as an additional material cost because a large amount of lubricant is required for satisfactory results.
It might be contemplated to use a solvent dilution of the lubricant but the practice would require a further process for dilution and need application of the lubricant with a greater frequency. Furthermore, when a silicone or a wax is used as the surface lubricant, the lubricant stuck to the surface of the product must be removed with a sanding paper or the like in order that the product may be receptive to a coating composition, with the result that the bonded product cannot be immediately coated. This is also a serious disadvantage.
It is, therefore, obvious that if a property that will permit easy removal of cured emulsion adhesive from the press or coater can be built into the adhesive itself and, at the same time, the bonding strength of the adhesive be improved, all of the above-mentioned problems will be solved at once and the utility of the emulsion adhesive be further expanded.